1) Field of the Invention
The present invention relates to a cylindrical composite paperboard cushion core and a process for producing the same. More particularly, the present invention relates to a cylindrical composite paperboard cushion core provided with an outer cushioning layer formed from an expanded paper sheet with a low density and having a high reusability and a satisfactory combustibility.
2) Description of the Related Art
It is known that paperboard tubes, thermoplastic resin tubes, for example, polypropylene resin tubes and polyvinyl chloride resin tubes, or composite tubes in which cushioning sheets are wound on the outer surfaces of the above-mentioned tubes, are usable as a winding core for paper sheets, viscose films (cellophane), thermoplastic resin films, various tapes, metallic foils and fabrics.
As a special winding core for photographic printing paper sheets and for winding few layers, there is a cushion paperboard core comprising a cylindrical paperboard substrate and a foamed thermoplastic resin cushion sheet attached to an outer surface of the cylindrical substrate. This type of cushion paperboard core is effectively utilized to prevent a formation of undesirable stepwise marks on the sheet wound around the core. The stepwise marks are derived from a terminal edge of the wound sheet placed on the outer surface of the core. The terminal edge forms a stepwise difference in level on the outer surface of the core. The cushioning layer allows the terminal edge of the wound sheet to be embedded in the cushioning layer so as to eliminate the stepwise difference and thus the formation of the stepwise marks on the wound sheet can be prevented.
Also, even when the sheet, for example, viscose film, is wound around a core under uneven tension, the cushioning layer effectively prevents a formation of undesirable uneven strain on the sheet.
As the conventional cushion sheet, usually foamed synthetic polymer films or sheets, for example, foamed polystyrene sheets or foamed polyethylene sheets, having a density of 0.08 to 0.16 g/cm.sup.3, are employed, because these foamed sheets exhibit a satisfactory cushioning effect and processability.
For example, when a compressive stress of 1 kgf/cm.sup.2 is applied to a foamed polystyrene sheet having a density of 0.09 g/cm.sup.3 in accordance with the static compression testing method of Japanese Industrial Standard (JIS) Z 0234, a compressive strain of about 40% is generated in the foamed sheet. Thus the foamed sheet exhibits a high cushioning effect. However, the conventional foamed cushion sheets are disadvantageous in that they have a high resistance to natural decomposition, and when burnt, harmful combustion gas and smoke are generated so as to pollute the environment. Also, the conventional foamed synthetic polymer cushion sheets are difficult to reuse.
As a conventional paperboard material having a high cushioning effect, a corrugated paperboard cushion sheet, especially, a one side-corrugated paperboard cushion sheet, is known. However, this conventional paperboard cushion sheet is disadvantageous in that processability is poor and when used as a cylindrical winding core, the surface of the paperboard sheet is too rough and uneven.
A conventional nonwoven fabric having a low density exhibits a high cushioning property. However, when employed to produce a cylindrical paperboard core, the nonwoven fabric exhibits a poor processability due to a poor mechanical strength thereof and a low resistance to ply separation.
JP-B-52-39,924 discloses a process for producing a cushioning paperboard substrate having a density of 0.37 to 0.67 g/cm.sup.3. In this process, a paperboard sheet is formed from a pulp slurry containing fine, porous inorganic particles.
Also, JP-B-55-18,116 discloses a porous paperboard plate having a density of 0.5 g/cm.sup.3 and produced by using expandable microcapsules. This paperboard plate is useful as a vibration diaphragm for a speaker. However, a paper sheet having a very low density of 0.1 to 0.4 g/cm.sup.3 which is comparable to that of conventional foamed polystyrene sheet, has not previously been known.